1. Field of the Invention
The present invention relates to a method and apparatus for control of crystal growth, and more specifically to controlling a temperature distribution near a solidification interface during crystal growth by the Czochralski method.
2. Description of Related Art
The well-known Czochralski method of growing crystals involves contacting a seed crystal with a melt of the crystal material contained in a heated crucible. The growth of large, substantially defect-free crystals by this and other methods remains to this day a difficult task, due to complications brought about by temperature gradients and convective eddies within the melt.
Previous attempts at controlling the crystal growth process to improve the quality of the crystals produced have primarily involved controlling process parameters such as crucible or crystal rotation speeds, crystal pulling rate, and total heat supplied to the crucible. Application of magnetic fields has also been used in attempting to suppress convective eddies within the melt. A further approach directed to suppressing or eliminating convection cells below a solidification interface has been to provide a baffle or other solid object in the crucible in the vicinity of the solidification front. All of the above approaches share the disadvantage that the controls tend to produce only bulk effects within the melt, and generally do not have any substantial effect on controlling the solidification interface as the crystal is pulled, which is possibly the most critical aspect of crystal growth, in terms of minimizing or substantially eliminating defects formed when growing large crystals.
Optical or electro-optical monitoring of the melt at the melt-crystal interface and controlled cooling of the pulled crystal are other known approaches directed to improving the quality of the crystal end product. External surface heating of the melt to maintain a predetermined temperature just above the melting temperature of the material has also been proposed. None of these approaches are believed to provide adequate control over the thermal conditions in the melt in the immediate vicinity of the solidification front of the pulled crystal.
It is therefore a principal object of the present invention to provide an apparatus whereby direct control of the thermal conditions in the vicinity of the solidification front or interface may be achieved.
It is an additional object of the present invention to provide a method for controlling the thermal conditions and radial temperature distribution in a melt in the vicinity of the solidification front of the crystal being pulled.
It is a further object of the present invention to provide an apparatus having a plurality of temperature measuring devices and an associated plurality of independently controllable heaters disposed at a location in the vicinity of the solidification front of the crystal being pulled.
It is a further object of the present invention to provide an apparatus which will yield crystals grown by the Czochralski method having improved size, yield and quality.